Along with swapping out the air filter and changing the oil, replacing the spark plugs is one of the few things left that require service on a modern gasoline engine. Some newer than the twenty-year old junk classics in our garage have negated even this task with spark plugs made of space-age materials that can live with their head stuck in a combustion chamber for 100K miles or more. The first and most obvious task of the spark plug is to light the gasoline and air mixture aflame when the piston reaches the top of its compression stroke. The resulting burn pushes the piston back down in the cylinder. So it goes. The second and less obvious function of a spark plug is equally important.

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He's Mr. Hundred and One
The less discussed function of the spark plug is to transfer heat away from the combustion chamber by acting as a heat exchanger. A spark plug produces no heat on its own, but rather wicks combustion heat through itself into the metal of the hole it's screwed into, and ultimately to the engine coolant surrounding that metal. Spark plugs are rated from cold to hot based on the speed at which they can transfer heat away from the combustion chamber. The correct heat range lets combustion byproducts burn away and the spark plug cleans itself.

Hot, No Wait Cold
Talk of cold and hot spark plugs is a bit counterintuitive. A spark plug in the colder heat range transfers heat quicker. A spark plug in the hotter heat range transfers heat slower. Using a spark plug too hot in heat range can bring excessive temperature, pre-ignition, and in severe cases a melting of the plug electrode. Metal parts flying around in the combustion chamber are very bad. Using a spark plug that's too cold in heat range can cause crud to build up on the electrode, which can lead to decreased spark efficiency.

Choose Wisely
The right spark plugs are designed specifically to work with the engine they're being screwed into. Spark plug manufacturers make this choice easy. Make. Model. Year. Engine. Presto! The next thing to check is the gap. While most conventional spark plugs come pre-gapped, it's a safe bet to consult the service or owner's manual for the correct gap and check with a feeler gauge before installing - with exception! Some newer and wicked expensive plugs can easily be ruined if gapped. Do not attempt to gap Iridium or similar spark plugs. There's usually something to the effect of do not attempt to gap printed on the side of the box.

How Often?
As far as when to change them, there is no reason to do it too often or wait too long. Every 30,000 miles is a good baseline for conventional copper core spark plugs, but keep in mind that all engines will have different requirements. Some newer vehicles will never require the average driver to even think about the spark plugs. While swapping out your spark plugs is relatively simple, there are a few classic screwups to avoid. We've put together a few tips compiled from years of shade free mechanical mishaps. Add some if you have some.

Start with a cool engine and a disconnected battery. Work one spark plug at a time. Clean the area around the spark plug boot or coil pack so crud doesn't fall into the hole left by spark plug. On wire equipped engines remove one spark plug boot by pulling on the boot itself. Boot pliers can help. Do NOT pull on the wire! Modern vehicles will have an ignition coil pack per cylinder arrangement, sometimes bolted in. Remove the fastener and pull the coil pack off the plug.

Remove the spark plug using a spark plug socket, extension, and ratchet. The spark plug should come loose with moderate effort. Stop if the spark plug offers too much resistance. The threads of the cylinder head can come out with the spark plug. This is rare but it can happen. We're not going to run how to install a helicoil thread insert until later.

Spark plug sockets have rubber inserts to hold the plug in place to make life easier. Lift the spark plug free of the hole. Be careful to keep debris from entering cylinder. Stuffing a clean rag in the hole is added safety.

If a spark plug socket isn't available then a length of hose over the end of the insulator can be used to spin the plug out of the hole. This trick works putting a plug into the hole as well. A universal joint can also help for tricky routing.

Check the new spark plug gap with a feeler gauge. Use the numbers on the gauge to match the recommended gap. Not too loose, not too tight. You should be able to feel the gauge contact both electrodes. Wire gauges are the most accurate, but we've never had any trouble using blade feeler gauges.

To adjust the gap use the adjuster on the gauge or Sears 4Way pocket screwdriver to gently bend open the outer electrode. Check that electrode surfaces are parallel to each other. To reduce the gap, lightly tap the spark plug on a level surface. Check the gap again

Put the spark plug back into the socket and thread it in by hand. The spark plug should offer little resistance. Stop immediately if it doesn't thread in like butter. Start over. A cross-threaded spark plug can be an expensive mistake. Turn the spark plug until it seats.

Use a wrench to tighten the spark plug enough to compress the washer against the cylinder head. Using a torque wrench is the best bet for the final twist. Barring that, turn the spark plug around a quarter turn after the gasket feels flattened. No more. Never over tighten spark plugs.

Reinstall the spark plug boot or coil pack. Check that the boot or coil pack is fully seated against the plug. It should be able to feel it click into place. Move onto the next plug!